In mixed model fixed repeating scheduling we want to balance run time and changeover time, that is we want to determine how much to run and the time interval between successive runs.  Some refer to this a Every Product Every Interval (EPEI).  Here’s the math:

A = Available Time (*% Uptime)
L = Production Load = sum run time for all members of the product family
= Sum (average demand for product i * cycle time for product i).  Note that this doesn’t include changeover time.
C = Total Changeover Time = sum of changeover times needed to accomplish one changeover for each product in family

PFTR = (A – L) / C

Example:

Available time is 2 shifts of 8 hours and 10% unplanned downtime
We have 4 products in the family with average daily demand of 14 A’s, 6 B’s, 4 C’s and 2 D
Cycle time for a unit is the same for all products, and is 30 minutes per piece
Changeover time between each product is 60 minutes

So:

A = 2 shifts/ day * 8 hrs/shift * 0.90 = 960 minutes * 0.90 = 864 minutes/day of effective available time
L = (14 + 6 + 4 + 2) * 30 minutes per piece = 780 minutes/day of production load or run time
C = 4 products * 1 hour changeover = 4 hours  = 240 minutes/family
PFTR then is (864 – 780) / 240 =  0.35 or a EPE Interval of 2.85 days.
Our run sizes would be 40 A’s, 17 B’s, 12 C’s and 6 D’s.

Here’s another example:

Number of different parts running through an injection molding center = 20
Setups take 1 hour between each mold change
20 days in a month, 16 hours in a day

Available Time = 20 day/month * 16 hours /day = 320 hr/month
Load or run time = sum or all cycle times to produce a month of demand = 280 hrs/ month
Time available to do change overs = 320-280 = 40 hours a month
Number of cycles per month = 40/20 = 20

Repeat Interval = 10 days, so make 10 days worth of each part every 10 days.